Extruded synthetic resinous foams are highly desirable for many applications including thermal insulation, decorative purposes, packaging and the like. One particularly important application for styrene polymer foams is in the field of thermal insulation. In this application, it is desirable that the insulating value of the foam be maintained for as long a period as possible and that the foam have dimensional stability. The present invention relates to polymer foams which are the so-called "extruded foams". The extruded foams generally have a fairly uniform cell size and are thus particularly useful for thermal insulation.
Another important characteristic of polymer foam is the density of the foam which generally may vary from about 1 to about 5 or more pounds per cubic foot. Densities of greater than about 2 pounds per cubic foot for polystyrene foams are common, and many such materials are commercially available. The relationship between density and cell size is an important consideration when the foamed product is to be utilized for thermal insulation. It generally is difficult to prepare foamed structures having low density (i.e., less than two pounds per cubic foot) while maintaining the desirable small cell sizes. Generally, as the density of the structure is decreased, the cell size becomes relatively larger, and attempts to obtain low density materials with relatively small cell size by controlling the amount of blowing agent and the temperature of the extrudate have not been completely successful. As the cell sizes are reduced, the material tends to have a higher density.
For a considerable period of time, styrene polymer foams have been extruded employing various halocarbons such as methyl chloride or ethyl chloride alone as blowing agents or as mixtures with chlorofluorocarbons (generally referred to as CFCs) such as dichlorodifluoromethane. Procedures which have utilized methyl chloride or ethyl chloride either alone or in combination with other blowing agents generally require that the extruded material be aged for a period of time sufficient to permit the methyl chloride or ethyl chloride to leave the cells and for air to enter by an appropriate diffusion process through the cell walls.
U.S. Pat. No. 4,393,016 describes the preparation of styrene polymer foams utilizing a mixture of ethyl chloride, methyl chloride and dichlorodifluoromethane as the blowing agents. U.S. Pat. No. 4,451,417 describes the use of a mixture of ethyl chloride and dichlorodifluoromethane.
In recent years, there has been significant pressure applied by the environmentalists and the government for the reduction, if not a complete ban, on the use of the chlorofluorocarbons (CFCs) for applications including aerosols, refrigerants, foam-blowing agents and specialty solvents within the electronics and aerospace industries. Examples of chlorofluorocarbons (CFCs) which have been utilized for these purposes include CFC-11 which is chlorotrifluoromethane, CFC-12 which is dichlorodifluoromethane, and CFC-113 which is 1,2,2-trifluoro-1,1,2-trichloroethane. Some attempts have been made in the past to replace the CFCs with hydrocarbons such as butane or inert gases such as carbon dioxide. For example, a blowing agent mixture of carbon dioxide and an alkane is described in U.S. Pat. Nos. 4,344,710 and 4,424,287. Other suggestions have been made in the prior art for the replacement or partial replacement of the CFCs which contain no hydrogen atoms with fluorohydrocarbons or chlorofluorohydrocarbons which, as the name applies, contain at least one hydrogen. These materials have been generally referred to as "soft CFCs", "HCFCs" and "HFCs".
A particular advantage of the HCFCs and HFCs or soft CFCs is that the ozone depletion potentials of these materials are significantly less than the ozone depletion potential of the CFCs. The ozone depletion potential is a relative measure of the capability of the material to destroy the ozone layer in the stratosphere. HCFCs and HFCs such as chlorodifluoromethane (F-22), 1,1-dichloro-2,2,2-trifluoroethane (F-123), 1-chloro-1,1-difluoroethane (F-142b), 1,1,1,2-tetrafluoroethane (F-134a), and 1,1-dichloro-1-fluoroethane (F-141b) have been found to have reduced ozone depletion potentials and may be acceptable substitutes for the CFCs. British Patent 1,537,421 describes styrene polymer foam bodies which have been extruded from mixtures utilizing as blowing agent, at least one compound of the formula EQU R.sub.1 --CF.sub.2 --R.sub.2
in which R.sub.1 is methyl, ethyl, chloromethyl, fluoromethyl, chlorofluoromethyl, difluoromethyl or trifluoromethyl, and R.sub.2 is hydrogen, chloro, fluoro, trifluoromethyl or methyl with the proviso that the compound contains not more than 3 carbon atoms, and if the compound contains only 2 fluorine atoms, it must contain 3 carbon atoms. The cells of the foam bodies prepared utilizing these blowing agents are reported as being substantially uniform and having a size of from 0.1 to 1.2 millimeters. The densities of the foam bodies are reported to be from 1 to 5 pounds per cubic foot. In one embodiment, one or more of the above blowing agents is combined with a second blowing agent having a permeability through the polymer of greater than 0.017 times that of nitrogen. Examples of such second blowing agents include methyl chloride, ethyl chloride, chlorodifluoromethane, fluorochloromethane and 1,1-difluoroethane.
British Patent 1,562,026 describes the preparation of styrene foams utilizing blowing agents similar to those described in British Patent 1,537,421 with the exception that R.sub.1 is dichloromethyl.
The use of a mixture of carbon dioxide, ethyl chloride and dichlorodifluoromethane (F-12), or 1-chloro-1,1-difluoroethane (F-142b) or mixtures thereof is described in U.S. Pat. No. 4,636,527. Nucleating agents may be incorporated into the foamable mixture to reduce the primary cell size. Examples of nucleating agents include talc, calcium silicate, indigo, etc.
The use of mixtures of blowing agents for preparing expanded closed cell foams of polystyrene resin also is subject to U.S. Pat. No. 3,960,792. The foaming agent mixture is one which has a diffusion rate through the polystyrene resin which is about 0.75 to 6 times the diffusion rate of air through polystyrene resin. Specific examples of such mixtures listed in the patent include methyl chloride and dichlorodifluoromethane; methyl chloride, neopentane and dichlorodifluoromethane; methyl chloride, difluorodichloromethane and dichlorotetrafluoroethane. The foaming agent mixture utilized in the formation of the polystyrene must be a non-solvent for the polystyrene resin.
U.S. Pat. No. 3,770,668 describes polystyrene foam bodies containing a plurality of closed gas-containing cells wherein the foam contains the specified amount of dichlorodifluoromethane, trichlorofluoromethane or 1,2-dichlorotetrafluoroethane, the average cell size is from about 0.1 to 0.45 millimeter, and the density of the foam body is from about 1.4 to 1.8 pounds per cubic foot. The foaming agent of polystyrene foams contains in addition to the chlorofluorocarbons mentioned above, an additional halocarbon selected from methyl chloride, ethyl chloride, vinyl chloride and mixtures thereof.
The general procedure utilized in the preparation of extruded synthetic resinous foam bodies generally involves the following steps. An alkenyl-substituted aromatic resin such as a polystyrene resin is heat plastified and one or more fluid blowing agents is incorporated and thoroughly mixed into the plastified resin under conditions which permit thorough mixing of the volatile blowing agent into the plastified resin and preventing foaming of the mixture. The intimate mixture of resin and blowing agents which may contain other optional additives including, for example, nucleating agents, flame-retardants for the foam, plasticizer, etc., is cooled, and the pressure on the mixture is reduced which results in foaming of the mixture and formation of the desirable foamed body. Foam bodies having desirable properties are obtained by extruding the cooled plastified mixture of resin and blowing agents into a region of lower pressure. In some foam extrusion processes, the foamable mixture is extruded into a vacuum chamber so that the expansion of the foam is accomplished under subatmospheric pressure. Examples of vacuum foam extrusion apparatuses and methods may be found in U.S. Pat. Nos. 3,584,108; 3,169,272; and 3,822,331. One of the difficulties when utilizing vacuum extrusion technology relates to the curing and extraction of the material from a vacuum chamber, specially when the material is delicate or fragile such as styrene foam in the form of large boards or billets. A solution to this problem involves the use of an incline barometric leg as described in U.S. Pat. Nos. 3,704,083 and 4,044,084. Further modifications and improvements in extrusion apparatus utilizing a large barometric leg which when evacuated is essentially filled with water, but which includes a vacuum chamber at its upper end into which the extrudate passes from the die for expansion is described in U.S. Pat. No. 4,199,320.